During sporulation, Bacillus thuringiensis (hereafter B.t.) produces proteinaceous crystals which are lethal to a variety of insect larvae. The proteins contained in the crystal, after being ingested by susceptible insect larvae, are transformed into biologically active moieties by proteases present in the insect gut. The crystal proteins are highly potent at destroying the gut's epithelium, and even nanogram amounts are capable of killing susceptible larvae. Some of the major insect pests in agriculture and forestry are species of the order Lepidoptera, which are known to be susceptible to B.t. toxins.
These crystal proteins have been grouped into four classes based on their host range and sequence homologies: Lepidoptera-specific (I), Lepidoptera/Diptera-specific (II), Coleoptera-specific (III), and Diptera-specific (IV) (Hofte, H. and H. R. Whiteley 1989. "Insecticidal Crystal Proteins of Bacillus thuringiensis." Microbiol. Rev. 53:242-255). Significant amino-acid similarities exist between the crystal proteins of the different classes with the carboxy-terminal half of the crystal proteins containing most of the conserved sequences. Five well-defined regions are conserved among most of the known crystal proteins; these are located in the N-terminal half of the protein, which is responsible for toxicity (Hofte, H. and H. R. Whiteley. 1989. "Insecticidal Crystal Proteins of Bacillus thuringiensis." Microbiol. Rev. 53:242-255). One exception is CytA, a 28 kDa cytolytic toxin from B. thuringiensis subsp. israelensis which has no detectable sequence identity with the other crystal proteins (Hofte, H. and H. R. Whiteley. 1989. "Insecticidal Crystal Proteins of Bacillus thuringiensis." Microbiol. Rev. 53:242-255).
The majority of the crystal proteins and all Class I lepidopteran-specific crystal proteins are synthesized as 130-140 kDa protoxins, which are then proteolytically cleaved in the insect midgut to 65-70 kDa active toxins. Some crystal proteins, Classes II and III, are produced as 65-70 kDa toxins. The only crystal protein which falls outside these two size ranges is CytA, one of five crystal proteins from dipteran-specific Bacillus thuringiensis subsp. israelensis. However, CytA has a different mode of action from other crystal proteins (Thomas, W. E., and D. J. Ellar. 1983. "Mechanism of action of Bacillus thuringiensis var. israelensis insecticidal .delta.-endotoxin." FEBS Lett. 154:362-367) and also has recently been reported to not be essential for mosquitocidal activity (Delecluse et al. 1991. "Deletion by in vivo recombination shows that the 28-kilodalton cytolytic polypeptide from Bacillus thuringiensis subsp. israelensis is not essential for mosquitocidal activity." J. Bacteriol. 173:3374-3381).